What is a Barlow lens?
Steve says: “A Barlow lens is an optical tube containing lens elements that diverge the light passing through them.
Inserting a Barlow lens into the light path of any type of telescope increases the effective focal length, usually by doubling it, although some designs have an even greater effect.
You can calculate the magnification of a telescope by dividing its focal length by the focal length of the eyepiece.
This means that an increase in the effective focal length of the telescope results in an increase in magnification.
So using a Barlow lens effectively doubles the number of eyepieces that you have in your collection!”
Email your astronomy queries to Steve at firstname.lastname@example.org and they could get answered in a future issue of BBC Sky at Night Magazine.